No it isn't wrong, like ptak30 says it's just easier to write that. In speech it sounds awkward but written it's fine, obviously not for a formal text though, unless of course you are stating a specific time.

Yes, the message was sent by a Liverpudlian and I wanted to use the am in my email sentence so that it sounds more casual and friendly. I didn't want to sound like a language learner using the easy way: "in the morning". So I will use "I will see you thursday a.m." without the "on".
x thanks